1. Field of the Invention
The present invention relates to a photovoltaic device such as a solar cell and a sensor which has a high conversion efficiency from light to electricity and allows little deterioration of the conversion efficiency with time even after long outdoor use, a manufacturing method of the same, a photovoltaic device integrated with a building material and a power-generating apparatus.
2. Related Background Art
Various photovoltaic devices have already been used as independent power sources for electric appliances and energy sources substituting for systematic electric power sources. Considering the cost required for generating unit electric power, however, the photovoltaic devices are still too expensive for use as substitute energy sources for the systematic electric power sources. Research is therefore actively being made to develop new photovoltaic devices.
As for technologies related to materials for photovoltaic devices, for example, research is made for crystalline materials that consist of single crystal or polycrystal silicon, amorphous silicon (a-Si), microcrystalline silicon (.mu.c-Si) and the so-called thin film materials that use compound semiconductors and on.
Speaking of solar cells that use microcrystalline silicon, Mat. Res. Soc. Symp. Proc. Vol. 420, p.3, 1996, J. Meier, H. Keppner, A. Shah, et al. reported that a solar cell obtained by the plasma CVD method using VHF (70 MHz) exhibited a photoelectric conversion efficiency of 7.7% with no deterioration by light. Furthermore, this literature reported that an initial photoelectric conversion efficiency of 13.1% was obtained with a solar cell which was manufactured by laminating amorphous silicon and microcrystalline silicon.
Furthermore, Drafts for the Spring Meeting of the Japan Society of Applied Physics Vol. 2, p.764, Article 30 p-B-4, reported a technology to form concavities and convexities on a surface of polycrystalline silicon.
Moreover, U.S. Pat. No. 4,419,533 discloses a technology to prepare a metallic reflecting layer so as to have concavities and convexities on a back surface of joined semiconductor layers, and dispose a barrier layer of zinc oxide on the joined semiconductor layers to prevent elements of the reflecting layer from being dispersed into the superposed semiconductor layers.
Japanese Patent Application Laid-Open No. 5-218469 discloses a technology to prepare a metallic reflecting layer so as to be flat, and form concavities and convexities by etching a surface of a barrier layer with an aqueous solution, thereby enclosing light.
While research is made to develop technologies to enhance conversion efficiencies, there has been reported, as a technology to form joined layers of thin film semiconductors at a low cost, a manufacturing technology that successively prepares laminated semiconductor layers on substrates of rolled stainless steel (Roll-to-Roll method) using a microwave for accelerating a deposition rate.
A most important key to decide whether or not photovoltaic devices are to be adopted is whether or not an amount of electricity to be obtained counterbalances amounts of materials used, a cost required for manufacturing, an area for installation, an appearance, etc. The conversion efficiency from light to electricity is not always the most important key to decide whether or not photovoltaic devices are to be used. For this reason, attention is paid to thin film type of photovoltaic devices that are rather low in conversion efficiencies thereof but can be manufactured at costs overwhelmingly lower than those of crystal type photovoltaic devices that have higher conversion efficiencies.
Accordingly, the problem imposed on photovoltaic devices is how to reduce a total power cost (cost/W). From this viewpoint, thin film type photovoltaic devices of amorphous silicon can be manufactured at a cost lower than that of any other type of photovoltaic devices. Furthermore, the Roll-to-Roll method permits lowering the manufacturing cost.
However, amorphous silicon type semiconductor materials that are typically represented by a-Si and a-SiGe are problematic since they exhibit a phenomenon which allows photoelectric conversion efficiencies thereof to be degraded when irradiated for long term with light corresponding to sunlight (deterioration by light). Though photovoltaic devices that have laminated structures of a-Si/a-SiGe/a-SiGe, a-Si/a-Si/a-SiGe, a-SiC/a-SiGe/a-SiGe and so on have been proposed as attempts to suppress the phenomenon of deterioration by light, these attempts are still insufficient as compared with crystalline silicon type photovoltaic devices. In addition, it is not easy to lower manufacturing costs of the photovoltaic devices having the laminated structures since expensive germane (GeH.sub.4) gas is used for forming a layer of a-SiGe.
As an attempt to suppress the phenomenon of deterioration by light, examinations are made on a photovoltaic device that uses microcrystalline silicon type materials. Though this photovoltaic device is quite free from the deterioration by light, it has a film thickness of 3.6 .mu.m, a short-circuit current of 25. 4 mA/cm.sup.2 and a photoelectric conversion efficiency that is still as low as 7.7%. Though a-Si/.mu.c-Si type laminated solar cell exhibits an initial photoelectric conversion efficiency of 13.1%, it is problematic in that an a-Si layer disposed on a side of incidence is remarkably deteriorated by light. Furthermore, it is problematic in that it uses a .mu.c-Si layer which is as thick as 3.6 .mu.m and allows deposition at a rate as low as 1.2 .ANG./sec, thereby requiring a layer formation time on the order of 8 hours and not being industrially practical.
Moreover, a technology reported by Drafts for the Spring Meeting of the Japan Society of Applied Physics Vol. 2, p. 764, Article 30 p-B-4, and the similar technologies that form concavities and convexities on surfaces of polycrystalline silicon, pose a problem that only limited kinds of materials are usable since these technologies require films as thick as 5 .mu.m and high manufacturing temperatures.